I still don't understand the objection. Sure, the target would try to defend itself. But the Klingons are firing multiple photon torpedoes at the target - strategic weapons intended to devastate entire cities and perhaps planets. Essentially, they are attacking an enemy star system, which just happens to consist of swirling light effects instead of simple vacuum. Attacking an enemy star system is a perfectly valid military maneuver in Star Trek, and a single starship (let alone three) should be capable of it in light of TOS precedent.

Now, DS9 shows that an attack on a properly defended star system may turn into a slaughter even if you have a hundred ships. But the Klingons wouldn't know what sort of defenses the enemy had (none were visible but total lack thereof was inconceivable). Which is probably exactly why they took such a cautious approach, firing from a considerable standoff distance and waiting to see what happened to their volley.

We later learn that V'Ger really is a spacecraft a few dozen kilometers long. Multiple photon torpedoes should have made short work of that, and the impressive-looking forcefield didn't seem to do much to stop them, either. The attack only failed because there existed an unexpected superdefense that could make the photon torpedoes disappear in mid-flight, plus a superoffense that could destroy the attackers.